This invention relates to the field of integrated circuit processing. More particularly, this invention relates to methods for application of an under bump metallization.
Flip-chip integrated circuits typically have bond pads that align with bond pads on a substrate such as a circuit board or package. The bond pads of the integrated circuit are connected to the corresponding bond pads on the substrate with electrical connections called bumps. In some bumping processes, such as solder bumping processes, an under bump metallization is initially provided on the integrated circuit bond pads.
The under bump metallization is typically used for any one or more of a number of different features, including keeping various materials separated, where the different materials may not be compatible with each other if they are allowed to come into direct contact with each other. Other features include those such as wettability, adhesion, and corrosion resistance.
The under bump metallization is typically formed with a physical vapor deposition process, such as sputtering or evaporation. However, these processes tend to be relatively expensive.
What is needed, therefore, is a method for forming under bump metallization.
The above and other needs are met by a method for providing under bump metallization on a substrate. Trenches are formed in the substrate, and a layer of first electrically conductive material is formed over the substrate. The layer of the first electrically conductive material substantially fills the trenches and substantially covers the substrate between the trenches in a contiguous sheet. The layer of the first electrically conductive material is thinned to an end point where the layer of the first electrically conductive material is substantially reduced in thickness, but still forms the contiguous sheet between the trenches.
A layer of photoresist is applied over the layer of the first electrically conductive material to define openings. A second electrically conductive material is deposited into the openings. The photoresist layer is removed, and the layer of the first electrically conductive material in the contiguous sheet between the trenches is removed to isolate the first electrically conductive material in the trenches.
Because the layer of the first electrically conductive material is not completely removed in the areas between the trenches, the first electrically conductive material may be used as an electrode for the electroplate deposition of the second electrically conductive material. Thus, the under bump metallization can be produced in a more economical manner. If the layer of the first electrically conductive material were to be thinned to the point where the first electrically conductive material was only left in the trenches, then it would not be feasible to used the layer of the first electrically conductive material as an electrode, and thus it would further not be feasible to electroplate the second electrically conductive material.
In various preferred embodiment of the invention, a barrier layer is first formed over the substrate. The barrier layer is most preferably a composite layer of tantalum and tantalum nitride. A seed layer is preferably formed over the barrier layer. Preferably, the seed layer is formed of the first electrically conductive material, which is most preferably copper. The second electrically conductive material is preferably nickel, which isolates the copper from other material which may degrade the copper, or which may be degraded by the copper.
In an especially preferred embodiment, a third electrically conductive material is deposited in the openings in the photoresist layer. Most preferably, the third electrically conductive material is either gold or solder. The gold and solder protect the nickel. However, if the third electrically conductive material is solder, then the second electrically conductive material of nickel may not be needed.
The layer of the first electrically conductive material is preferably formed by electroplating, using the underlying layers as electrodes. The layer of the first electrically conductive material is preferably thinned using a chemical mechanical polishing process.